1. Field of the Invention
The present invention relates to an electrical cable, in particular for low-tension power transmission or for telecommunications, this cable comprising a coating which has fire-resistance properties and is capable of keeping its electrical insulation properties unchanged when said cable is in the presence of moisture.
2. Description of Related Art
Besides retarding the propagation of fire, cable coatings defined as being xe2x80x9cfire resistantxe2x80x9d should, in the presence of fire, afford a very low emission of fumes, a low level of emission of noxious gases, and should be self-extinguishing. Combustion-resistant cables are assessed for use in closed environments by means of performance tests against industrial standards which define the limits and provide the methodology for cable flammability tests. Examples of these standards are ASTM 2863 and ASTM E622; IEEE-383, IEEE-1202 (devised by the xe2x80x9cInstitute of Electrical and Electronics Engineersxe2x80x9d, New York, USA); UL-1581 and UL-44 (xe2x80x9cUnderwriters Laboratories Inc.xe2x80x9d, Northbrook, Ill., USA); CSA C22.2 0.3 (xe2x80x9cCanadian Standard Associationxe2x80x9d, Toronto, Canada)
Typical characteristics of moisture-resistant coatings are a limited absorption of water and the maintenance of constant electrical properties, even in the presence of moisture; an example of a reference standard for said characteristics is the abovementioned reference UL-1581.
Coated cables which simultaneously have fire-resistance properties and moisture-resistance properties are also described, according to the xe2x80x9cUS Electric National Codexe2x80x9d, as xe2x80x9cRHHxe2x80x9d, xe2x80x9cRHW/2xe2x80x9d or xe2x80x9cXHHWxe2x80x9d cables. The abbreviation xe2x80x9cRHHxe2x80x9d indicates a single conductor having an insulator which is acceptable for use in a dry location at 90xc2x0 C.; the abbreviation xe2x80x9cRHW/2xe2x80x9d indicates a single conductor having an insulator which is acceptable for use in a dry or wet location at 90xc2x0 C.; and the abbreviation xe2x80x9cXHHWxe2x80x9d indicates a single conductor having an insulator which is acceptable for use in a dry location at 90xc2x0 C. and in a wet location at 75xc2x0 C.
The use of halogenated additives (compounds based on fluorine, chlorine or bromine) which are capable of giving fire-resistant properties to the polymer which forms the coating, or of polymers based on halogenated compounds (for example polyvinyl chloride) having fire-resistant properties per se, has the drawback that the decomposition products of halogenated compounds are toxic, as a result of which the use of such materials, especially for uses in closed locations, is not recommended.
Alternatively, of the substances capable of imparting fire-resistant properties to coatings for cables, inorganic oxides are particularly valued, for example aluminum, magnesium, titanium and bismuth oxides, in particular in hydrated form. These compounds generally need to be xe2x80x9ccompatibilizedxe2x80x9d with the polymer matrix by means of special additives which are capable of bonding both with the inorganic charge and with the polymer matrix. However, these inorganic oxides also have strong hydrophilicity properties and, since these substances are added in relatively large amounts in order to obtain the desired fire-resistant effect, the coating may absorb considerable amounts of water, with a consequent reduction in its electrical insulation properties.
Currently, the best method for overcoming this drawback is to add to the mixture which forms the coating silane-based compounds, which, besides improving the compatibility between the inorganic charge and the polymer matrix, make it possible to maintain good properties of dielectric insulation after exposure of the cable to a wet environment; see, for example, the information reported in U.S. Pat. No. 4,385,136xe2x80x94Pat. No. Re31, 992xe2x80x94(col. 4, lines 49-67). These silane compounds are also described in many commercial catalogues and brochures from numerous companies, including Union Carbidexe2x80x94xe2x80x9cSilane coupling agent in mineral reinforced Elastomerxe2x80x9d (1983), Hxc3xclsxe2x80x94xe2x80x9cApplications of organofunctional silanesxe2x80x9d (1990).
However, the Applicant has observed that the use of such compounds has the drawback that the resulting mixture, precisely because of the presence of silanes, tends to adhere to the surface of the metal conductor in contact with the inner layer. This drawback reduces the so-called xe2x80x9cstrippabilityxe2x80x9d of the cable, thus creating problems in cable laying operations. The Applicant has also observed that, in the cables which are commercially available, in particular those for telecommunications, in order to overcome the abovementioned drawback, the conductor is coated with a separating strip (generally based on polyester), the specific purpose of which is to prevent the mixture from bonding to the conductor; the fire-resistant coating containing the silane compound is then extruded over this strip. It is clear that this strip-insertion operation includes the introduction of an additional stage in the processing of the cable and in its application.
U.S. Pat. No. 4,317,765 describes the use of maleic anhydride for compatibilizing an inorganic charge with a polyolefin, in particular polyethylene. That patent points out that polyolefin, inorganic charge and anhydride must be made to react simultaneously in order to obtain materials with good mechanical strength properties (col. 6, lines 41-45); in particular (col. 7, line 54xe2x80x94col. 8, line 3), mixing the inorganic charge with polyethylene which has already been reacted with maleic anhydride produces a material with poor mechanical properties.
Patent JP 63-225,641 describes the use of a dicarboxylic acid or anhydride derivative in a mixture containing a polymer and an inorganic charge, in particular magnesium hydroxide, for the purpose of preventing this magnesium hydroxide from reacting with atmospheric moisture and carbon dioxide and being converted into carbonate, thus causing the formation of a whitish compound on the surface of the cable coating.
Neither of these documents mentions the problem of maintaining the dielectric insulation properties after exposure of the cable to a wet environment, nor the problem of strippability mentioned above.
GB 2,294,801 discloses a cable having an inner sheath made of polyethylene (PE) or polypropylene (PP) in contact with the conductive wire and an outer sheath made of fire retardant material, such as xe2x80x9clow smoke zero halogenxe2x80x9d rubber or PVC. The PE or PP employed as materials for the inner layer are intended as waterproof materials. However, no mention is made about the fire retardant properties of the said inner layer. As a matter of fact, the presence of the inner layer consisting essentially of a polyolefynic material would substantially reduce the overall fire resistance properties of the cable""s sheath.
The Applicant has observed that the properties of fire resistance and of insulation resistance in the presence of moisture are difficult to reconcile in a single cable coating, since the fire resistance is increased the larger the amount of inorganic charge present in the coating, whereas the insulation resistance in the presence of moisture reduces as the inorganic charge in the coating increases. The Applicant has also observed that the presence of suitable coupling agents in the mixture which forms the coating, while improving the insulation resistance of the coating, lowers its capacity to absorb water, thus reducing its fire-resistance properties with respect to a coating not containing said coupling agent.
The Applicant has now found that it is possible to construct a cable which simultaneously has the desired properties of fire-resistance and of insulation resistance in the presence of moisture, in which the coating of said cable is formed of a double layer, the outer layer of this coating being constructed so as mainly to impart to said cable said fire-resistance properties and the inner layer being constructed so as to impart properties of insulation resistance in the presence of moisture, while giving a substantial contribution to the overall fire-resistant properties of the cable.
In the present description, when the inner layer is said to xe2x80x9csubstantially contribute to the overall fire-resistance properties of the cablexe2x80x9d it is intended that although the fire-resistant properties are mainly imparted by the outer layer, nevertheless the inner layer is also endowed with substantial fire-resistance properties, differently from the known waterproof coating layers having no such characteristics.
In particular, this result may be obtained when the inner layer of said coating comprises a polymer matrix with an inorganic charge dispersed in this matrix, so as to provide substantial fire-resitant properties, and a predetermined amount of coupling agent such as to provide the desired insulation-resistance properties in the presence of moisture; and the outer layer comprises a base polymer matrix and an inorganic charge dispersed in this matrix in an amount such as to provide the cable with the desired fire-resistance properties.
The Applicant has observed that when the coupling agent present in the inner layer is a polyolefin compound containing at least one unsaturation and at least one carboxyl group in the polymer chain (identified in the remainder of the present description by the term xe2x80x9ccarboxylated polyolefinxe2x80x9d), the resulting cable not only has the desired insulation-resistance properties in the presence of moisture but is also readily strippable.
The Applicant has also observed that if a polymer composition for coating cables does not contain such an additive or other coupling agent known in the art, or at any rate contains it in amounts lower than the abovementioned predetermined amount, when said cable is in the presence of moisture this coating is able to absorb a certain amount of water, thereby increasing the fire resistance of this cable.
The Applicant has moreover found that with the abovementioned double-layer structure of the coating, the outer layer being the one which mainly imparts the fire resistance, it is possible to add to this outer layer an amount of inorganic charge which is greater than the amount of the inner layer, without this having a negative impact on the dielectric properties of the coating, which are, in any case, guaranteed by the presence of the inner layer; in this way, the fire resistance of the outer layer is increased both owing to the larger amount of inorganic charge present and owing to the increased capacity of said inorganic charge to absorb water (that is to say more inorganic charge capable of absorbing water) On the other hand, by endowing the inner layer with substantial fire-resistant properties, thus contributing to the overall fire-resistant properties of the cable, the applicant has found that it is possible to advantageously reduce the thickness of the outer layer of the coating, with respect to the thickness of an outer layer enveloping an inner layer having no fire-resistant properties.
In this respect, the Applicant has also found that an advantageous embodiment of the present invention is. obtained by suitably selecting the kind of mineral charge to be added in the two layers, in such a way to further improve the moisture resistance of the cable coating at high temperatures.